The present invention relates generally to optical amplification and more particularly to excitation of optical parametric oscillators.
An optical parametric oscillator (OPO) is a nonlinear device employing a nonlinear optical resonance phenomenon to convert incident photons into photon pairs when optically excited at a power per unit area above a threshold level, the threshold level being a characteristic of the nonlinear material and the wavelengths. This device is usually practiced in one of two forms: the doubly resonant oscillator (DRO) in which both of the generated optical beams are resonated, or the singly resonant oscillator (SRO) in which only one of the generated optical beams is resonated.
The development of optical parametric oscillators for the commercial market requires the achievement of several simultaneous features. These features include primarily a low pump intensity threshold and output stability. It is desirable to have the capability to drive an OPO by a variety of pump lasers, including in particular small, low power, inexpensive lasers, such as the Helium-Neon (HeNe) laser, the air-cooled Argon ion laser, and the semiconductor diode laser. OPOs pumped with continuous wave (CW) lasers present two central difficulties. Since a high intensity must be used, the first problem has been that a costly, high average power laser is required. The second problem is that the pump laser frequency must be extremely well controlled to obtain a stable continuous output. While high stability has been achieved in some laboratory laser systems, technical advances are needed which both reduce the required pump laser average power and satisfy the stability requirements for a feasible commercial system.
In the experimental literature beginning with the invention of the OPO, four experimental groups report working on continuous OPOs through 1974. These groups are from Bell Labs, from Stanford, from Rice, and from Karlsruhe. In none of this work was true continuous operation achieved because there was high sensitivity of the configuration to frequency fluctuations.
Interest was revived in CW OPOs in 1987 when two groups used triply resonant OPOs for quantum "squeezed state" experiments: L-A. Wu et al. "Squeezed states of light from an optical parametric oscillator", J. Opt. Soc. Am., Vol. 4, No. 10, pp. 1465-1475 (1987), and P. Grangier et al, "Squeezed-light-enhanced polarization interferometer", Phys. Rev. Lett., Vol 59, No. 19, pp. 2153-2156 (1987). The triply resonant OPO is a DRO with the addition of a buildup cavity to increase the intensity of the source laser at the OPO crystal. Using this approach, operation was possible with pump laser average powers of 30 mW and 150 mW, respectively. To achieve this buildup, extensive stabilization measures were taken. The Wu et al. laser was locked with a broadband electronic stabilization system to an external Fabry-Perot resonator. Both groups then had to stabilize the length of the OPO resonator to the fluctuating output of the pump laser in order to excite the triply resonant OPO. Neither group was particularly sensitive to the instability in the power generated by the OPO (the first group even worked below the OPO threshold).
In 1989, Nabors et al. showed that the pulsing problems of CW OPOs can be solved by using the highly stable nonplanar ring YAG laser as the pump (C. D. Nabors et al., "Efficient, single-axial-mode operation of a monolithic MgO:LiNbO.sub.3 optical parametric oscillator", Opt. Lett., Vol. 14, No. 20, pp. 1134-1136 (1989); and C. D.. Nabors et al., "Coherence properties of a doubly-resonant monolithic optical parametric oscillator", Journ. Opt. Soc. Am., Vol. B7, pp. 815-820 (1990)). In order to reach threshold with the available pump power, they selected the doubly resonant oscillator (DRO) configuration in which both the signal and the idler waves are enhanced by a resonant cavity surrounding the nonlinear crystal of the OPO. Since the DRO has a substantially lower threshold than the SRO, they were able to obtain oscillation.
Both of these approaches were doubly resonant oscillators. However, the preferred configuration from the point of view of a user is the singly resonant OPO because of its capability for smooth tuning. The doubly resonant oscillator has a tuning problem because all three of the interacting waves have a defined longitudinal mode structure, leading to an over-constrained system. The pump wave longitudinal mode structure is defined by the pump laser cavity. Both the signal and the idler wave longitudinal mode structures are defined by the OPO resonant cavity. The result of this situation is that the DRO cannot be tuned without mode hops which depend on the temperature of the crystal and the length of the cavity. These hops can be unacceptably large. In the SRO configuration, only one of the signal beam or the idler beam is resonated, removing the over-constrained condition, and allowing continuous tuning.
Unfortunately, the high threshold of the SRO eliminates from consideration many of the most desirable pump lasers. What is needed is a singly-resonant parametric oscillator capable of being driven stably by a low-powered optical source.
There are other structures which have relevance to the invention described hereinafter. An OPO has been operated inside a resonant cavity of a pump laser. See E. O. Ammann et al., "Efficient internal optical parametric oscillation", Appl. Phys. Lett., Vol. 16, pp. 309-312 (1970). By this means, the pump power at the nonlinear crystal is made larger than the laser output power, as in the approach described hereinafter. However, since there is no external cavity, there is clearly no need for injection locking. The intracavity approach was first analyzed by M. K. Oshman et al., "Theory of Optical Parametric Oscillation internal to the laser cavity", IEEE Journ. Quant. Elect., Vol. QE-4 pp. 491-502 (1968). This approach helps reach threshold for some pump laser types, but it does not address the pulsation instabilities for CW operation, which are caused by frequency instabilities. It is also incompatible with the semiconductor diode laser, since no space is available intracavity, and the cavity Q is so low that no significant improvement would be obtained by applying this approach to the external cavity diode laser. Even for the lasers where this approach is applicable, Oshman et al. have shown that the laser power cannot exceed three times the threshold, or CW operation becomes impossible due to induced instability. This constraint further limits the applicability of the intracavity approach.
Frequency doubling configurations which use the buildup cavity have been described in the following four articles: W. J. Kozlovsky et al., "Generation of 41 mW of blue radiation by frequency doubling of a GaAlAs diode laser", Appl. Phys Lett. Vol. 56, pp. 2291-2292 (1990); A. Hemmerich et al., "Seconddiodeharmonic generation and optical stabilization of a laser in an external ring resonator", Opt. Lett. Vol. 15 pp. 372-374 (1990); L. Goldberg et al., "Efficient generation at 421 nm by resonantly enhanced doubling of GaAlAs laser diode array emission", Appl. Phys. Lett., Vol. 55, pp. 218-220 (1989); and G. J. Dixon et al., "432 nm source based on efficient second-harmonic generation of GaAlAs diode-laser radiation in a self-locking external resonant cavity", Opt. Lett., Vol. 14, pp. 731-733 (1989).
All of these approaches used semiconductor diode lasers as the pump source, all of them performed the doubling operation with a resonator to build up the pump laser power and increase the efficiency of the doubling operation, and all employ locking to the source. However, with the object being to double an output frequency, there is no attention paid to the problems of multiple-wavelength mirror reflectivity and transmission, tuning, or three-beam phase matching, which are characteristic of an OPO. Dielectric mirrors are always very close to perfectly anti-reflection coated at the second harmonic of their primary reflection frequency. All of these researchers treat this as an advantage for extracting the second harmonic generated in the doubling crystal. There is no consideration of the needs for additional coatings to achieve the doubly or triply resonant cavity needed for an OPO.